The combustion of gaseous fuels requires mixing of the fuel gas with a source of oxidant. This source of oxidant is most commonly normal ambient air. "Oxygen-enriched" air is also obtainable by a variety of methods in known manner. Each gaseous fuel has lower and upper flammability limits which define the minimum and maximum fuel content of the fuel/oxidant mixture in order for combustion to occur. Depending on the gaseous fuel and the fuel concentration of the fuel/oxidant mixture, the resulting flame will possess a characteristic flame speed. The flame speed is the velocity at which the flame will propagate towards the source of the fuel/oxidant mixture supply. A stable flame front (i.e., leading edge of the flame) is established, when the velocity of the fuel/oxidant supply equals the flame speed.
The propagation of a flame front depends on the leading edge of the flame being able to raise the temperature of the fuel/oxidant mixture through which it is propagating to a minimum temperature (ignition temperature) in order for the chemical reaction of combustion to occur. If unable to continually raise the fuel/oxidant mixture to the ignition temperature, the flame front will establish itself at a point where it was last able to achieve the required temperature rise of the fuel/oxidant mixture.
The purpose of a "premix" burner is to provide a controlled flow of fuel/oxidant mixture to the area intended for the combustion process (flame zone). When the flow rate of the fuel/oxidant mixture through a burner reduces, the port velocity reduces. This typically causes the flame front to establish itself closer to the actual burner port(s). As the flame front anchors closer to the burner port(s), thermal energy from the base of the flame will heat up the material forming the burner port(s). The temperature resistance of the burner port material therefore is one limiting design feature for establishing a minimum allowable port velocity. When flame speed exceeds port velocity, the flame front will attempt to propagate through the burner port(s). Energy absorbed by the burner port material tends to "quench" the flame front. If the flame front can raise the fuel/oxidant mixture to ignition temperature despite the quenching effect of the port material, the flame might propagate through the port(s) and ignite the fuel/oxidant mixture upstream of the burner port(s). This undesirable condition is called flashback or pre-ignition. This condition can also occur if the burner port material itself gets hot enough to ignite the fuel/oxidant mixture upstream of the burner port(s). These design limitations restrict the ability of burners to accommodate a large turndown ratio (i.e., fuel input range), and hence, operating range for the burner.
Many conventional burners are not intended to operate as radiant bodies. Such burners are often formed from steel either with slot-shaped or circular ports. As such, they produce hot combustion gases, with insignificant radiant heat.
Radiant burners are preferred for a number of applications. They have the advantage of maintaining a lower flame temperature and hence, reducing the production of nitrous oxides.
Burners intended to be capable of operation as radiant bodies exist in a variety of forms, for example: solid clay ported ceramics manufactured by Hamilton Procelains and Schwank; fibrous permeable matrices manufactured by Alzeta Pyrocore and A. O. Smith.
The solid clay ported types are restricted to low levels of energy per unit surface area, are not capable of wide input rate turndown and possess relatively high thermal conductivity conducive to pre-ignition if overheated. They are therefore typically used for fixed input burners for use in open atmosphere.
The fibrous permeable matrix types allow the fuel/oxidant mixture to permeate only through the matrix of fibers. Therefore these types operate in a predominantly subcutaneous combustion mode. They possess only modest turndown capability because of the predominance of subcutaneous combustion operation and the inherent flame quenching that occurs especially at low fuel input rates. They can also suffer from extraordinary fuel and/or oxidant filtration requirements to prevent clogging.
U.S. Pat. No. 4,673,349 (Abe et al.) discloses a high temperature surface combustion burner plate made from a ceramic porous body with a plurality of throughholes. However, little teaching is given on the formation of the throughholes, and it simply refers to forming these either by pins during the molding, or subsequently by drilling.
Neither technique provides a satisfactory method for the quick, economic production of burners. Pins can become stuck in the burner or rupture it during removal, while drilling is very time-consuming. What is therefore desired is a method of manufacturing a burner capable of withstanding elevated temperatures so as to act as a radiant body which method is simple and reliable. The burner should possess a low thermal conductivity yet have sufficient flame thermal quenching characteristics to minimize or eliminate flashback/pre-ignition. The method should provide passageways which are not prone to clogging so as to only require reasonable filtration. The method should enable burners of a variety of shapes or dimensions to be produced to accommodate scaling of energy input range or adaptation to various combustion chamber orientations. The burner should be economical to produce.